Proximity switch with magnetic field-sensitive sensor

ABSTRACT

A proximity switch with a magnetic field-sensitive sensor is formed of limbs of a U-shaped permanent magnet with a vertical direction of magnetization. A magnetic flux-free region, in which the sensor is fitted, is formed by three like poles. When a flat ferromagnetic triggering part is brought close to the two poles of the limbs of the U in a plane parallel to the base of the U, a switching signal which is triggered from the sensor by the cancellation of the magnetic flux-free region can be evaluated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a proximity switch having a magneticfield-sensitive sensor.

Conventional mechanical switches are increasingly being displaced, inparticular in motor vehicle applications, by non-contacting proximityswitches having a magnetic field-sensitive sensor, for example a Hallsensor or a magneto resistor. In principle, Hall sensors are composed ofa semiconductor layer, in particular one made of silicon, which issupplied with a constant current. The constant current is influenced bya magnetic field component at right angles to the layer and the sensorsupplies a Hall voltage which may be evaluated and which is proportionalto the applied magnetic field strength. At present, Hall sensors areexpediently used in the form of an integrated circuit which alreadycontains an evaluation circuit which is suitable for evaluating theswitching state.

2. Description of the Prior Art

German Offenlegungschrift 39 01 678 discloses positioning the magneticfield-sensitive sensor in a magnetic field-free zone of a specialpermanent magnet arrangement. In order to achieve a high temperaturestability, the permanent magnet in the known proximity switch istherefore configured in such a way that an annular pole surface results.In this case, driving by a flat ferromagnetic triggering part is carriedout at right angles to the pole plane, the magnetic field-free zone notbeing canceled when the triggering part is brought close but only whenmoving away from the sensor.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a proximity switchhaving a magnetic field-sensitive sensor which is particularly simplyconstructed and is very simple to produce.

According to the invention, this object is achieved by a proximityswitch having a magnetic field-sensitive sensor,

having a U-shaped permanent magnet with vertical direction ofmagnetization,

a magnetic flux-free region, in which the magnetic field-sensitivesensor is fitted, being formed between the limbs of the U by three like,mutually repelling poles,

having a flat ferromagnetic triggering part which can be brought closeabove the limbs of the U in a plane parallel to the base of the U, andwherein

a switching signal can be evaluated which is triggered from the sensorby the cancellation of the magnetic flux-free region when the triggeringpart is brought close to the two poles of the limbs of the U.

The invention is explained in further detail below with reference to anexemplary embodiment represented in the single FIGURE of the drawing.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows, in a side view, a magnet system of a proximity switchaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A U-shaped magnet 10 with a vertical direction of magnetization isrepresented in the FIGURE. Flux lines are shown at 9. There is amagnetic flux-free region 11 between the limbs 10A and 10B of the U,since the three north poles repel one another. A magneticfield-sensitive sensor 12 is fitted in this region 11. When aferromagnetic triggering part 14 is brought close to the U-shaped magnet10, above the magnet, in the plane indicated, the two lateral northpoles are partly shielded or their magnetic field is deflected. Themagnetic flux of the central north pole is then able to penetratebetter, that is to say it flows through the magnetic field sensor 12 andthus triggers a switching signal.

The rectangular flux-free region 11 shown in the drawing figure has alongitudinal extent (horizontal in the drawing) which is substantiallyperpendicular to a magnetic pole axis (vertical axis running through Nand S poles in the drawing) of each of the limbs.

The arrangement represented is particularly simply constructed andspace-saving. It is also advantageous that shielding of the sensor fromthree sides results from the particular geometric configuration of themagnet system. A reduced magnetic susceptibility to faults of the sensoris of significance, for example in the case of applications of theproximity switch in motor vehicle central locking systems.

A switch which was constructed by way of a trial already had a switchingdistance of 1.5 mm given dimensions of in each case 4 mm for thethickness of the magnet and for the width of the limbs of the U and forthe region between the limbs. This switching distance can readily beoptimized further by suitable determination of the dimensions of the Umagnets, that is to say it can be increased. A relatively greatswitching distance is desired, since in practice tolerances occurbetween the proximity switch, which is incorporated in a lock, forexample, and the triggering part, the tolerances being able to lead toan excessive distance between the triggering part which is brought closeand the switch in the event of too small a switching distance.

In a proximity switch according to the invention, the proximity switchcomprises a housing for the permanent magnet, shown in dashed lines at13, which is composed of thermoplastic. The permanent magnet is composedof a plastic-bonded permanent magnet powder. The permanent magnet 10 andthe housing 13 are produced using two-component injection molding; Thehousing is molded directly onto the permanent magnet. The permanentmagnet is magnetized after the encapsulation, and may be producedparticularly cost effectively. For instance, in this arrangement the Umagnet is injection molded first and is subsequently encapsulated by thehousing. The two components are therefore molded one after the other inone and the same injection molding machine. This can be carried outparticularly simply if different plastics are used for the housing andthe permanent magnet. Even in the case of the same plastics, thetheoretical risk of resoftening of a previously injection-moldedcomponent can be countered by appropriate adaptation of theinjection-molding parameters and molding cycle times, so that theindividual components can be molded one after the other and can bemolded on without the components sticking to one another.

The essential steps of the production process can, for example, becarried out in such a way that a copper tape is stamped and subsequentlyelectroplated, that Hall chips having two to three connections are, forexample, directly bonded on one side, and that this populated coppertape subsequently runs into an injection molding machine. There, theplastic-bonded permanent magnet powder, for example polyester granules,and then the housing with molded-in circuit board are then molded oneafter the other. The magnetic field sensor 12 is either molded into thehousing at the same time or is subsequently inserted.

Although various minor changes and modifications might be proposed bythosed skilled in the art, it will be understood that my wish is toinclude within the claims of the patent warranted hereon all suchchanges and modifications as reasonably come within my contribution tothe art.

I claim as my invention:
 1. A proximity switch, comprising:a U-shapedpermanent magnet with a vertical direction of magnetization, saidpermanent magnet defining a magnetic flux-free region formed by threelike mutually repelling poles and positioned between limbs of theU-shaped magnet; a magnetic field-sensitive sensor positioned in saidmagnetic flux-free region between said limbs and inwardly of terminatingends of said limbs, said flux-free region having a longitudinal extentsubstantially perpendicular to a magnetic pole axis of each of saidlimbs; a flat ferromagnetic triggering part positionable closely abovesaid limbs of the U-shaped magnet in a plane parallel to a base of theU-shaped magnet; and a switching signal triggered from the sensor by atleast partial cancellation of the magnetic flux-free region when thetriggering part is brought close to poles of said limbs of the U-shapedmagnet, the switching signal being capable of evaluation to indicateactivation of the proximity switch.
 2. The proximity switch according toclaim 1 wherein the permanent magnet is formed of a plastic-bondedpermanent magnet powder.
 3. The proximity switch according to claim 1wherein the permanent magnet is encapsulated in a housing together withsaid magnetic field-sensitive sensor so that said magneticfield-sensitive sensor is fixed in said magnetic flux-free region bysaid housing.
 4. A method for constructing a proximity switch,comprising the steps of:forming a U-shaped permanent magnet with avertical direction of magnetization with a plastic-bonded permanentmagnet powder, the U-shaped permanent magnet having a magnetic flux-freeregion between limbs of the U-shaped magnet caused by three likemutually repelling poles of the U-shaped permanent magnet, saidflux-free region having a longitudinal extent substantiallyperpendicular to a magnetic pole axis of each of said limbs; providing amagnetic field-sensitive sensor and positioning said magneticfield-sensitive sensor in said magnetic flux-free region between saidlimbs and inwardly of terminating ends of said limbs by encapsulatingthe U-shaped permanent magnet and the magnetic field-sensitive sensor ina housing; and dimensioning said housing such that a flat ferromagnetictriggering part can be brought close above said limbs of the U-shapedmagnet in a plane parallel to a base of the U-shaped magnet so that aswitching signal is triggered from the sensor by cancellation of themagnetic flux-free region when the triggering part is brought close topoles of said limbs of the U-shaped magnet, the switching signal beingcapable of evaluation to indicate activation of the proximity switch. 5.The method according to claim 4 including the steps of forming thehousing around the permanent magnet by an injection molding directlyonto the permanent magnet, and magnetizing the permanent magnet afterencapsulation.
 6. The method according to claim 4 wherein the magneticfield-sensitive sensor is molded into the housing at the same time thehousing is being molded.
 7. The method according to claim 4 wherein themagnetic field-sensitive sensor is inserted into the housing after thehousing has been injection molded.
 8. A proximity switch system,comprising:a U-shaped permanent magnet with a vertical direction ofmagnetization, said permanent magnet defining a magnetic flux-freeregion formed by three like mutually repelling poles and positionedbetween limbs of the U-shaped magnet, a first of the like poles being ina base of the U-shaped magnet between the limbs and second and thirdones of the like poles being at ends of said limbs, said flux-freeregion having a longitudinal extent substantially perpendicular to amagnetic pole axis of each of said limbs; a magnetic field-sensitivesensor positioned in said magnetic flux-free region between said limbsand inwardly of terminating ends of said limbs; a housing encapsulatingthe permanent magnet and the sensor and positioning the sensor relativeto the magnet; a flat ferromagnetic triggering part positionable closelyabove said limbs of the U-shaped magnet in a plane parallel to the baseof the U-shaped magnet; and a switching signal triggered from the sensorby cancellation of the magnetic flux-free region when the triggeringpart is brought close to poles of said limbs of the U-shaped magnet, theswitching signal being capable of evaluation to indicate activation ofthe proximity switch.